1. Field of the Invention
This invention relates to a noise suppressing circuit, and particularly to a circuit for deriving a video signal with noise being suppressed in a video signal recording/reproducing apparatus such as a video tape recorder or the like.
2. Description of the Prior Art
Various types of noise suppressing circuits are known which are used for suppressing noise included in a video signal. FIG. 11 and FIG. 12 show conventional noise suppressing circuits used for suppressing noise included in a video signal processed in a reproducing system such as a video tape recorder.
In a circuit of FIG. 11, high frequency components are derived by a circuit comprising a low pass filter 2 and a subtractor 3 from a video signal incoming through a terminal 1. Then amplitude is limited by a limiter 4 to derive high frequency small-amplitude components of the video signal. These components are regarded the same as noise and are subtracted from the input video signal by way of a subtractor 5. As a result, a video signal whose noise has been suppressed is outputted through a terminal 6. Since this circuit performs noise suppression using information within one horizontal scanning period, this circuit will be referred to as an in-line noise suppressing circuit.
On the other hand, in a circuit of FIG. 12, vertical high frequency components of a video signal incoming through an input terminal 1 are obtained by 1-H delay circuit 7 and a subtractor 3, and noise is obtained ty limiting amplitude by a limiter 4 and an attenuator having a gain of approximately 1/2. This noise is subtracted from the input video signal by a subtractor 5 to obtain a video signal whose noise has been suppressed. Since this circuit performs noise suppression using the fact that a video signal has vertical correlation (correlation between two points which are spaced 1-H period), this circuit will be referred to as a vertical correlation noise canceller.
The noise suppressing effect by these noise suppressing circuits is determined by the passing level of the limiter 4 and by the level of the output from the limiter 4 which is to be subtracted from the input video signal, and the noise suppressing effect will increase as the passing level of the limiter 4 is made larger and larger.
The in-line noise suppressing circuit shown in FIG. 11 exhibits maximum noise suppressing effect when the gain of the limiter output and the gain of the input video signal are set as in a ratio of 1 to 1, i.e., a limiter output of 1 to an input videl signal of 1=. However, there occurs an unfavorable side effect that the small-amplitude details of the incoming video signal are lost. Furthermore, the vertical correlation noise canceller shown in FIG. 12 exhibits maximum noise suppressing effect when the gain of the limiter output and the gain of the input video signal are set as a ratio of (limiter output) (input video signal)=1/2: 1. However, this causes vertical signal components to be lost, and thus there occurs the unfavorable side effect that the picture is blurred in a vertical direction.
Therefore, in the conventional circuits the passing level of the above-mentioned limiter and the level of the limiter output to be subtracted from the input video signal are set in consideration of the balance with respect to the SN ratio of the video signal.
The recent development of magnetic tapes is remarkable, and various types have been developed whose SN ratio has been improved compared to conventional standard magnetic tapes. However, since the above-mentioned conventional circuits are arranged such that limiter level and limiter output gain are fixed irrespective of the SN ratio of the input video signal, when designing a video deck, the above-mentioned setting is performed so that satisfactory SN ratio is obtained when reproducing a standard tape.
In conventional video decks, there is a drawback that it is difficult to obtain high quality pictures by using high quality magnetic tape since the unfavorable side effects of dropout of the details of the small-amplitude input video signal and blur in vertical direction remain unchanged even if SN ratio is good enough when reproducing such a high quality magnetic tape.
In addition, there is a drawback that when using conventional circuits, the poor SN ratio will appear on the screen as it is in the case of poor SN ratio of the recorded video signal caused by incorrect tracking in the reproduce mode or by radio wave receiving state or the like.